Maintaining spatial stability utilizing common gain coefficient

ABSTRACT

In a system and method for maintaining the spatial stability of a sound field a background noise estimate may be estimated for each of a first signal and a second signal. A first gain coefficient may be calculated responsive to the first audio signal and the background noise estimate of the first audio signal. A second gain coefficient may be calculated responsive to the second signal and the background noise estimate of the second signal. The gain coefficients may be calculated using one or more gain coefficient calculators. A common gain coefficient may be selected from one of the first gain coefficient and the second gain coefficient. The selected common gain coefficient may be one that results in a least amount of audio signal modification and may be applied to each of the first signal and the second signal.

PRIORITY CLAIM

This application is a continuation of U.S. patent application Ser. No.13/753,211, filed Jan. 29, 2013, which is herein incorporated byreference.

BACKGROUND

1. Technical Field

The present disclosure relates to the field of processing sound fields.In particular, to a system and method for maintaining the spatialstability of a sound field utilizing a common gain coefficient.

2. Related Art

Stereo and multichannel audio configurations may be used for processinga sound field that is a spatial representation of an audibleenvironment. The processed sound field may be used to reproduce theaudible environment using audio transducers.

Many computing devices may have multiple integrated microphones used forrecording an audible environment associated with the computing deviceand communicating with other users. Computing devices typically usemultiple microphones to improve noise performance with noise suppressionprocesses. The noise suppression processes may result in the reductionor loss of spatial information. In many cases the noise suppressionprocessing may result in a single, or mono, output signal that has nospatial information.

BRIEF DESCRIPTION OF DRAWINGS

The system may be better understood with reference to the followingdrawings and description. The components in the figures are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the disclosure. Moreover, in the figures, likereferenced numerals designate corresponding parts throughout thedifferent views.

Other systems, methods, features and advantages will be, or will become,apparent to one with skill in the art upon examination of the followingfigures and detailed description. It is intended that all suchadditional systems, methods, features and advantages be included withthis description and be protected by the following claims.

FIG. 1 is a schematic representation of a system for maintaining thespatial stability of a sound field.

FIG. 2 is a further schematic representation of a system for maintainingthe spatial stability of the sound field.

FIG. 3 is flow diagram representing a method for maintaining the spatialstability of the sound field.

FIG. 4 is a schematic representation of a system for maintaining thespatial stability of a sound field.

DETAILED DESCRIPTION

In a system and method for maintaining the spatial stability of a soundfield a background noise estimate may be estimated for each of a firstaudio signal and a second audio signal. A first gain coefficient may becalculated responsive to the first audio signal and the background noiseestimate of the first audio signal. Gain coefficients may be calculatedusing a gain coefficient calculator that may include one or more of anautomatic gain controller, a noise suppressor and an echo canceller. Asecond gain coefficient may be calculated responsive to the second audiosignal and the background noise estimate of the second audio signal. Thesecond gain coefficient may be calculated with one or more gaincoefficient calculations similar to those applied to the first signal. Acommon gain coefficient may be selected from one of the first gaincoefficient and the second gain coefficient. Selecting a common gaincoefficient may comprise selecting a gain coefficient from the firstgain coefficient and the second gain coefficient that will result in aleast amount of audio signal modification. The selected common gaincoefficient may be applied to each of the first audio signal and thesecond audio signal.

FIG. 1 is a schematic representation of a system for maintaining thespatial stability of a sound field. Two or more inputs 102, or audioinputs 102, may receive the sound field. Stereo and multichannel inputconfigurations may be utilized for processing the sound field that is aspatial representation of an audible environment associated. The audibleenvironment may be associated with microphones on a local computingdevice or a remote computing device. The remote computing device maytransmit audio signals 120 to the local computing device that mayutilize the received audio signals 120 as inputs 102. Many audibleenvironments associated with the inputs 102 may include undesirablecontent that may be mitigated by processing the received sound field.Microphones that are arranged in a far field configuration typicallyreceive more undesirable content (a.k.a. noise) than microphones in anear field configuration. Far field configurations may include, forexample, a hands free phone, a conference phone and microphones embeddedinto an automobile. Far field configurations are capable of receiving asound field that represents the spatial environment associated with themicrophones. Near field configurations typically place the microphone inclose proximity to a user. Undesirable content may be mitigated in bothnear and far field configurations by processing the received soundfield.

Processing that may mitigate undesirable content received in the soundfield may include a gain coefficient calculator 106. The gaincoefficient calculator 106 may comprise one or more of a noisesuppressor 110 and an echo canceller 112. The echo canceller 112, noisesuppressor 110 and other audio processing processes may calculate one ormore gain coefficients. Each respective gain coefficient may be appliedindividually or a composite gain coefficient may be applied to processthe sound field using a gain coefficient applier 118.

The echo canceller 112 mitigates echoes caused by signal feedbackbetween two or more communication devices. Signal feedback occurs whenan audio transducer on a first communication device reproduces thesignal received from a second communication device and subsequently themicrophones on the first communication device recapture the reproducedsignal. The recaptured signal may be transmitted to the secondcommunication device where the recaptured signal may be perceived as anecho of the previously transmitted signal. The echo canceller 112 maydetect when the signal has been recaptured and attempt to suppress therecaptured signal. Many different types of echo cancellers 112 maymitigate echoes by calculating one or more gain coefficients that, whenapplied to the signals received by the microphone, suppress the echoes.In one example implementation, the echo suppressor 112 may calculategain coefficients using a coherence calculation between near and farsignals disclosed in U.S. Pat. No. 8,036,879, which is incorporatedherein by reference, except that in the event of any inconsistentdisclosure or definition from the present specification, the disclosureor definition herein shall be deemed to prevail.

When the microphone, or source of input 102, and an audio transducer areclose in proximity, the echo canceller 112 may determine that a largeamount of suppression may mitigate the signal produced by the audiotransducer from dominating, or coupling with, the microphone. The echocanceller 112 may calculated large gain coefficients to mitigate thecoupling. The large gain coefficients may result in a gating effectwhere the communication device effectively supports only half duplexcommunication. Half duplex communication may occur when thecommunication channel allows for reliable communication fromalternatively either the far side or near side but not bothsimultaneously. The large gain coefficients may suppress the couplingbut may also suppress all content, including desired voice contentresulting in half duplex communication.

Background noise is another type of undesirable signal content that maybe mitigated by processing the input 102. Many different types of noisesuppressor 110 techniques may mitigate background noise. An exemplarynoise suppressor 110 is a recursive Wiener filter. The Wienersuppression gain G_(i,k), or gain coefficient, is defined as:

$\begin{matrix}{G_{i,k} = {\frac{{\hat{SNR}}_{{priori}_{i,k}}}{{\hat{SNR}}_{{priori}_{i,k}} + 1}.}} & (1)\end{matrix}$

Where S{circumflex over (N)}R_(priori) _(i,k) is the a priori SNRestimate and is calculated recursively by:

S{circumflex over (N)}R _(priori) _(i,k) =G _(i−1,k) S{circumflex over(N)}R _(post) _(i,k) −1.   (2)

Where S{circumflex over (N)}R_(priori) _(i,k) is the a posteriori SNRestimate given by:

$\begin{matrix}{{\hat{SNR}}_{{post}_{i,k}} = {\frac{{Y_{i,k}}^{2}}{{{\hat{N}}_{i,k}}^{2}}.}} & (3)\end{matrix}$

Where |{circumflex over (N)}_(i,k)| is a background noise estimate. Abackground noise estimator 104 may estimate the background noiseestimate. In one example implementation, the background noise estimate,or signal values, may be calculated using the background noiseestimation techniques disclosed in U.S. Pat. No. 7,844,453, which isincorporated herein by reference, except that in the event of anyinconsistent disclosure or definition from the present specification,the disclosure or definition herein shall be deemed to prevail. In otherimplementations, alternative background noise estimation techniques maybe used, such as, for example, a noise power estimation technique basedon minimum statistics.

An automatic gain controller 108 may calculate gain coefficients thatmay mitigate changing energy levels of the desired signal content. Forexample, the energy level of a user speaking into the microphone maychange over time as the microphone may change position relative to theuser. The gain coefficients calculated by the automatic gain controller108 may mitigate the perception of the microphone changing position whenapplied to the input 102. In one example implementation, the automaticgain controller 108 may calculate gain coefficients using the gaincontroller techniques disclosed in U.S. Pat. No. 8,116,485, which isincorporated herein by reference, except that in the event of anyinconsistent disclosure or definition from the present specification,the disclosure or definition herein shall be deemed to prevail.

The gain coefficient calculators 106 including the automatic gaincontroller 108, the noise suppressor 110 and the echo canceller 112described above may be responsive to a background noise estimategenerated by the background noise estimator 104. The automatic gaincontroller 108 may utilize the background noise estimate to calculategain coefficients that may be adjusted when signal energy exceeds thebackground noise estimate resulting in less background noise beingamplified. The echo canceller 112 may utilize the background noiseestimate to calculate gain coefficients when the echoes exceed thebackground noise estimate by a threshold. The background noise estimator104 may calculate a background noise estimate for each input 102.

When the inputs 102 are generated by physically separated microphones,or when two or more inputs 102 do not contain identical signal content,the background noise estimator 104 and the gain coefficient calculator106 may calculate different background noise estimates and gaincoefficients. Differences between the first gain coefficients applied tothe first input 102 and second gain coefficients applied to the secondinput 102 may cause a distortion in the spatial image when reproduced inthe output 116. Different gain coefficients applied to the first input102 and the second input 102 may result in a shifting spatial image thatmay be distracting to a listener.

A common gain coefficient selector 114 may mitigate some distortion inthe spatial image by selecting a first gain coefficient or a second gaincoefficient that may be applied to both the first input 102 and thesecond input 102. Applying the same gain coefficient to all inputs 102may mitigate distortions in the spatial image. The largest or thesmallest gain coefficient may be selected. Alternatively, a combinationof the gain coefficients may be calculated. For example, when the firstinput 102 contains a larger echo than the second input 102, an averagegain coefficient may be calculated to perceptibly remove the largerecho. In another alternative, the common gain coefficient selector 114may select the gain coefficient that will result in a least amount ofaudio signal modification. For example, the echo canceller 112 maycalculate a first gain coefficient for application to the first input102 that is larger than a second gain coefficient for application to thesecond input 102. The larger gain coefficient may result in a lowerenergy signal to reduce the amount of echo in the first input 102. Thecommon gain coefficient selector 114 may select the smaller gaincoefficient for application to both the first input 102 and the secondinput 102 resulting in less echo suppression while mitigatingdistortions in the spatial image. The common gain coefficient selector114 may utilize two or more inputs 102 where the selected gaincoefficient is applied to the two or more input 102. For example, thecommon gain coefficient selector 114 in addition to the first input 102and the second input 102 may process a third input 102 and theirrespective calculated gain coefficients.

The common gain coefficient selector 114 may not apply to all types ofgain coefficient calculators 106. For example, some types of noisereduction processes may require additional processing to mitigatedistortions in the spatial image. A first input 102 that containssignificant wind noise may have wind noise reduction applied while thesecond input 102 has no wind noise reduction applied. The common gaincoefficient selector 114 may not mitigate the wind noise that may bemore distorting than a shift in the spatial image. In oneimplementation, wind noise suppression gains (a.k.a. gain coefficients)may be calculated using the system for suppressing wind noise disclosedin U.S. Pat. No. 7,885,420, which is incorporated herein by reference,except that in the event of any inconsistent disclosure or definitionfrom the present specification, the disclosure or definition hereinshall be deemed to prevail. In another example, when the microphone andaudio transducer are coupled as described above resulting in a gatingeffect, the common gain coefficient selector 114 may not be utilizedbecause the audible artifacts associated with the coupling areperceptibly more distracting than distorting the spatial image.

A subband filter may process the received input 102 to extract frequencyinformation. The subband filter may be accomplished by various methods,such as a Fast Fourier Transform (FFT), critical filter bank, octavefilter band, or one-third octave filter bank. Alternatively, the subbandanalysis may include a time-based filter bank. The time-based filterbank may be composed of a bank of overlapping bandpass filters, wherethe center frequencies have non-linear spacing such as octave, 3^(rd)octave, bark, mel, or other spacing techniques. The one or more gaincoefficients may be calculated for each frequency bin or band of thesubband filter. The gain coefficients and the selected gain coefficientsmay be filtered, or smoothed, over time and/or frequency.

FIG. 3 is flow diagram representing a method for maintaining the spatialstability of the sound field. The method 300 may be, for example,implemented using the systems 100 and 200 described herein withreference to FIGS. 1 and 2. The method 300 may include the followingacts. Estimating a background noise estimate for each of a first audiosignal and a second audio signal 302. Calculating a first gaincoefficient responsive to the first audio signal and the backgroundnoise estimate of the first audio signal 304. The gain coefficients maybe calculated using a gain coefficient calculator that may include oneor more of an automatic gain controller, a noise suppressor and an echocanceller. Calculating a second gain coefficient responsive to thesecond audio signal and the background noise estimate of the secondaudio signal 306. The second gain coefficient may be calculated with oneor more of the same gain coefficient calculations similar to thoseapplied to the first signal. Selecting a common gain coefficient fromone of the first gain coefficient and the second gain coefficient 308.Selecting the common gain coefficient may comprise selecting a gaincoefficient from the first gain coefficient and the second gaincoefficient that will result in a least amount of audio signalmodification. Applying the selected common gain coefficient to each ofthe first audio signal and the second audio signal 310.

FIG. 2 is a further schematic representation of a system for maintainingthe spatial stability of a sound field. The system 200 comprises aprocessor 202, memory 204 (the contents of which are accessible by theprocessor 202) and an I/O interface 206. The memory 204 may storeinstructions which when executed using the processor 202 may cause thesystem 200 to render the functionality associated with the system formaintaining the spatial stability of the sound field as describedherein. In addition the memory 204 may store instructions which whenexecuted using the processor 202 may cause the system 200 to render thefunctionality associated with the background noise estimator 104, thegain coefficient calculator 106, the automatic gain controller 108, thenoise suppressor 110, the echo canceller 112, the common gaincoefficient selector 114 and the gain coefficient applier 118 describedherein. In addition, data structure, temporary variables and otherinformation may be stored in data storage 208.

The processor 202 may comprise a single processor or multiple processorsthat may be disposed on a single chip, on multiple devices ordistributed over more that one system. The processor 202 may be hardwarethat executes computer executable instructions or computer code embodiedin the memory 204 or in other memory to perform one or more features ofthe system. The processor 202 may include a general purpose processor, acentral processing unit (CPU), a graphics processing unit (GPU), anapplication specific integrated circuit (ASIC), a digital signalprocessor (DSP), a field programmable gate array (FPGA), a digitalcircuit, an analog circuit, a microcontroller, any other type ofprocessor, or any combination thereof.

The memory 204 may comprise a device for storing and retrieving data,processor executable instructions, or any combination thereof. Thememory 204 may include non-volatile and/or volatile memory, such as arandom access memory (RAM), a read-only memory (ROM), an erasableprogrammable read-only memory (EPROM), or a flash memory. The memory 204may comprise a single device or multiple devices that may be disposed onone or more dedicated memory devices or on a processor or other similardevice. Alternatively or in addition, the memory 204 may include anoptical, magnetic (hard-drive) or any other form of data storage device.

The memory 204 may store computer code, such as the background noiseestimator 104, the gain coefficient calculator 106, the automatic gaincontroller 108, the noise suppressor 110, the echo canceller 112, thecommon gain coefficient selector 114 and the gain coefficient applier118 as described herein. The computer code may include instructionsexecutable with the processor 202. The computer code may be written inany computer language, such as C, C++, assembly language, channelprogram code, and/or any combination of computer languages. The memory204 may store information in data structures including, for example, thegain coefficients.

The I/O interface 206 may be used to connect devices such as, forexample, the input 102 and output 116 to other components of the system200.

All of the disclosure, regardless of the particular implementationdescribed, is exemplary in nature, rather than limiting. The systems 100and 200 may include more, fewer, or different components thanillustrated in FIGS. 1 and 2. Furthermore, each one of the components ofsystems 100 and 200 may include more, fewer, or different elements thanis illustrated in FIGS. 1 and 2. Flags, data, databases, tables,entities, and other data structures may be separately stored andmanaged, may be incorporated into a single memory or database, may bedistributed, or may be logically and physically organized in manydifferent ways. The components may operate independently or be part of asame program or hardware. The components may be resident on separatehardware, such as separate removable circuit boards, or share commonhardware, such as a same memory and processor for implementinginstructions from the memory. Programs may be parts of a single program,separate programs, or distributed across several memories andprocessors.

The functions, acts or tasks illustrated in the figures or described maybe executed in response to one or more sets of logic or instructionsstored in or on computer readable media. The functions, acts or tasksare independent of the particular type of instructions set, storagemedia, processor or processing strategy and may be performed bysoftware, hardware, integrated circuits, firmware, micro code and thelike, operating alone or in combination. Likewise, processing strategiesmay include multiprocessing, multitasking, parallel processing,distributed processing, and/or any other type of processing. In oneembodiment, the instructions are stored on a removable media device forreading by local or remote systems. In other embodiments, the logic orinstructions are stored in a remote location for transfer through acomputer network or over telephone lines. In yet other embodiments, thelogic or instructions may be stored within a given computer such as, forexample, a CPU.

While various embodiments of the system and method for maintaining thespatial stability of a sound field have been described, it will beapparent to those of ordinary skill in the art that many moreembodiments and implementations are possible within the scope of thepresent invention. Accordingly, the invention is not to be restrictedexcept in light of the attached claims and their equivalents.

1. A computer implemented method for maintaining the spatial stabilityof a sound field comprising: estimating a background noise estimate foreach of a first audio signal and a second audio signal; calculating afirst gain coefficient responsive to the first audio signal and thebackground noise estimate of the first audio signal; calculating asecond gain coefficient responsive to the second audio signal and thebackground noise estimate of the second audio signal; selecting a commongain coefficient from the first gain coefficient and the second gaincoefficient; and applying the selected common gain coefficient to eachof the first audio signal and the second audio signal.
 2. The method formaintaining the spatial stability of a sound field of claim 1, whereselecting the common gain coefficient comprises selecting a gaincoefficient, from the first gain coefficient and the second gaincoefficient, that will result in a least amount of audio signalmodification.
 3. The method for maintaining the spatial stability of asound field of claim 1, where selecting the common gain coefficientcomprises selecting a gain coefficient, from the first gain coefficientand the second gain coefficient, that will result in a greatest amountof audio signal modification.
 4. The method for maintaining the spatialstability of a sound field of claim 1, where selecting the common gaincoefficient comprises averaging a gain coefficient, from the first gaincoefficient and the second gain coefficient, that will result in acombined average amount of audio signal modification.
 5. The method formaintaining the spatial stability of a sound field of claim 1, furthercomprising: estimating a background noise estimate for a third audiosignal; calculating a third gain coefficient responsive to the thirdaudio signal and the background noise estimate of the third audiosignal; selecting the common gain coefficient from the first gaincoefficient, the second gain coefficient and the third gain coefficient;and applying the selected common gain coefficient to each of the firstaudio signal, the second audio signal and the third audio signal.
 6. Themethod for maintaining the spatial stability of a sound field of claim1, where calculating each of the first gain coefficient and the secondgain coefficient includes one or more of a noise suppressioncalculation, an echo cancellation calculation and an automatic gaincontrol calculation.
 7. The method for maintaining the spatial stabilityof a sound field of claim 1, further comprising generating a set ofsub-bands for each of the first audio signal and the second audio signalusing a subband filter or a Fast Fourier Transform.
 8. The method formaintaining the spatial stability of a sound field of claim 1, furthercomprising generating a set of sub-bands for each of the first audiosignal and the second audio signal according to a critical, octave, mel,or bark band spacing technique.
 9. A system for maintaining the spatialstability of a sound field comprising: a background noise estimator toestimate a background noise estimate for each of a first audio signaland a second audio signal; a gain coefficient calculator to calculate afirst gain coefficient responsive to the first audio signal and thebackground noise estimate of the first audio signal and to calculate asecond gain coefficient responsive to the second audio signal and thebackground noise estimate of the second audio signal; a common gaincoefficient selector to select a common gain coefficient from the firstgain coefficient and the second gain coefficient; and a gain coefficientapplier to apply the selected common gain coefficient to each of thefirst audio signal and the second audio signal.
 10. The system formaintaining the spatial stability of a sound field of claim 9, where thecommon gain coefficient selector comprises selecting a gain coefficient,from the first gain coefficient and the second gain coefficient, thatwill result in a least amount of audio signal modification.
 11. Thesystem for maintaining the spatial stability of a sound field of claim9, where the common gain coefficient selector comprises selecting a gaincoefficient, from the first gain coefficient and the second gaincoefficient, that will result in a greatest amount of audio signalmodification.
 12. The system for maintaining the spatial stability of asound field of claim 9, where the common gain coefficient selectorcomprises averaging a gain coefficient, from the first gain coefficientand the second gain coefficient, that will result in a combined averageamount of audio signal modification.
 13. The system for maintaining thespatial stability of a sound field of claim 9, where: the backgroundnoise estimator further to estimate a background noise estimate for athird audio signal; the gain coefficient calculator further to calculatea third gain coefficient responsive to the third audio signal and thebackground noise estimate of the third audio signal; the common gaincoefficient selector further to select the common gain coefficient fromthe first gain coefficient, the second gain coefficient and the thirdgain coefficient; and the gain coefficient applier further to apply theselected common gain coefficient to each of the first audio signal, thesecond audio signal and the third audio signal.
 14. The system formaintaining the spatial stability of a sound field of claim 9, wherecalculating each of the first gain coefficient and the second gaincoefficient includes one or more of a noise suppression calculation, anecho cancellation calculation and an automatic gain control calculation.15. The system for maintaining the spatial stability of a sound field ofclaim 9, further comprising means to generate a set of sub-bands foreach of the first audio signal and the second audio signal using asubband filter or a Fast Fourier Transform.
 16. The system formaintaining the spatial stability of a sound field of claim 9, furthercomprising means to generate a set of sub-bands for each of the firstaudio signal and the second audio signal according to a critical,octave, mel, or bark band spacing technique.